Plant innate immunity against pathogens depends on a network of functionally interconnected genes involved in the regulation and execution of defense reactions (Glazebrook et al. (2003) Plant J. 34:217; Tsuda et al. (2009) PLoS Genet. 5:e1000772). A fundamental form of innate immunity in plants involves conserved microbe-associated molecular patterns (MAMPs) or general elicitors. MAMPs are recognized by pattern recognition receptors (PRRs) on the surface of plant cells. Upon pathogen recognition, PRRs activate a comprehensive set of defense reactions collectively referred to as pattern triggered immunity (PTI). Some pathogens have independently acquired the ability to evade PTI through the release of effector molecules, suppressing defense and thus enabling infection (effector triggered susceptibility, ETS). In this case, the pathogen is virulent and the host susceptible. Even in the face of ETS, plants can mount a weakened immune response, called basal defense, which limits pathogen spread. Basal defense typically cannot fully prevent disease. As a countermeasure to ETS, plants have also evolved the ability to recognize the presence of effectors by highly specific plant resistance (R) proteins, which mediate effector triggered immunity (ETI) resulting in incompatible interactions and leaving pathogens avirulent.
ETI, basal defense, and PTI pathways share some signaling components, such as reactive oxygen species, Ca2+, salicylic acid (SA) and jasmonic acid (JA) (Nimchuk et al. (2003) Ann. Rev. Genet. 37:579). The plant immune system can be subdivided into various defined sectors that can interact with each other. For example, distinct defense signaling sectors depend on early MAMP-activated MAP kinases or the messenger molecules SA or JA, and some of these sectors can interact in an additive or synergistic fashion during PTI, or in an antagonistic manner during ETI. The latter phenomenon can compensate if a defined sector is disabled due to interferences with a pathogen effector. A general review of chemical defense inducers can be found in Schreiber & Desveaux (2008) Plant Pathology J. 24:245.
Pesticides are commonly used in agriculture and horticulture for disease control. Current chemical pesticides, however, typically rely on direct antibiotic or biocidal activity, which often leads to undesirable toxic environmental side effects. For example, seven of the 10 most frequently used pesticides in California tomato production have potential acute toxic, carcinogenic, neurotoxic or groundwater-contaminating activities, or detrimental effects on human reproduction and development.